Chinese Patent Application No. 00800767.5 discloses a device that uses magnets to move the brush on planar aquarium windows, thus cleaning the window surface. The interior element of such a brush has an air chamber therein. After magnetic contact is broken, the interior element will float on the surface of the water so that it can be readily removed. As such, it must overcome considerable friction to move the two attractive elements. As the thickness of the aquarium window increases, the size of the magnetic material used inside the two elements increases accordingly. When the weight of said air chamber and the magnetic material are equal to the buoyance, the interior element will float in water and yet move smoothly on the surface of the window due to the magnetic influence of the exterior element. On the other hand, when the weight of said air chamber and the magnetic material is not equal to the buoyance, say heavier than the buoyance, the interior element will sink to the bottom of the water upon removal of the magnetic force. When the weight of said air chamber and the magnetic material is lighter than the buoyance, the higher buoyance makes the movement of the interior element more difficult, as known from Chinese Patent Application No. 2008000767 that also is Publication No. CN 1302182. Furthermore, a much stronger magnetic force is required to make the two elements attract while cleaning near the bottom of the aquarium to overcome the detachment of the interior and exterior elements. Moreover, the right angle shape of the bottom plate of the interior element makes it suitable only for cleaning the conventional rectangular aquarium, and is thus quite difficult to move on the curved window surface of aquariums which are available more and more on the market.